1. Field of the Invention
The present invention relates to a system for automatically controlling lift-augmentation devices of an aircraft during take-off.
2. Description of Related Art
It is known that, particularly for the sake of profitability, the airline companies seek to increase as far as possible the occupancy level of their airplanes, which, needless to say, has the consequence of increasing the mass of these airplanes. However, at certain airports, the landing and take-off runways may prove to be of insufficient length to allow airplanes exhibiting a very high mass to take off.
Thus the airplane manufacturers seek to reduce the runway length necessary for take-off, whatever the type and/or the mass of the airplane in question.
To that end, in order to enhance the performance on take-off, the wings of the airplane, especially of civil transport airplanes, are generally equipped with lift-augmentation devices (slats on the leading edge of the wing and flaps at the trailing edge) which can be deployed and retracted, which make it possible to augment substantially the lift generated at given incidence, when they are deployed, and also to push back the phenomenon of stalling. This augmentation in the lift has the consequence of reducing the flying speeds and, thus, of reducing the length of runway necessary for take-off.
Consequently, it is advantageous, during take-off, to deploy these lift-augmentation devices as much as possible in order to augment the lift and thus reduce the length of runway necessary for take-off.
However, the deployment of the lift-augmentation devices, while augmenting the lift, also augments the drag. This is because the profile of the wings is modified by the presence of these devices, which degrades its aerodynamic behavior as regards drag: its profile departs from the xe2x80x9cclean wingxe2x80x9d profile. This then entails a degradation in the angle of climb.
However, this angle of climb of the aircraft (when the latter has left the ground) has to comply with regulatory constraints. This is because aeronautical regulations stipulate a minimum angle of climb, with one engine assumed to have failed, this being done in order to ensure that an airplane, one of the engines of which has inadvertently failed, can complete its take-off under good safety conditions.
Consequently, in order to preserve a minimum angle of climb allowing take-off in complete safety, it is advantageous to deploy the lift-augmentation devices as little as possible in order to reduce the corresponding drag as much as possible.
It emerges clearly from the foregoing that the choice of the configuration of the lift-augmentation devices for the take-off phase results from a compromise between the runway length (favorable to extraction or deployment of the lift-augmentation devices) and the angle of climb (favorable to the retraction of said lift-augmentation devices). This choice, which is made by the pilot of the aircraft, is made on the basis of the take-off conditions (available runway length, altitude, temperature, mass of the airplane, presence of an obstacle, etc.). Once the pilot has made his choice, he configures the lift-augmentation devices by the use of a control lever into the configuration corresponding to this choice. The configuration thus chosen is kept throughout the entire take-off phase since, in the current state of aeronautical regulations, it is forbidden for the pilot to alter this configuration throughout the entire take-off phase, so as to allow him to concentrate exclusively on monitoring his trajectory and his flight parameters.
Consequently, in the current state of the regulations and of the technology, the choice of the position or configuration of the lift-augmentation devices results from a compromise between two contradictory requirements, which is made before the take-off phase and which is therefore not optimal.
Systems are known making it possible to improve the position of said lift-augmentation devices.
However, these known systems apply, in general, only in response to a disturbance (failure of an engine or gusts of wind especially) which alters the flight conditions of the airplane. Thus, by way of illustration, the document FR-2 425 380 describes a control system which, when an engine fails, acts automatically on the control surfaces so as to configure the airplane aerodynamically in such a way as to compensate for the effect of the loss of thrust on the aerodynamic characteristics of the wing. Moreover, the document EP-0 337 581 discloses a system which, in the event of gusts of wind during the approach phase, compensates for the loss of altitude and the pitch generated by the gusts, by increasing the speed of the airplane by increasing the thrust from the engines, taking account, particularly, of the position of the flaps.
Furthermore, the document U.S. Pat. No. 4,042,197 describes a device which is for the purpose, in the take-off and approach phases, of optimizing the position of the lift-augmentation flaps, as well as the thrust, in such a way as to reduce substantially the noise generated by this equipment. As far as the take-off phase is concerned, information on speed and on the raising of the landing gear (landing gear fully raised) are used to set the position of the lift-augmentation flaps. This information is compared with reference information (speed, distance since releasing the brakes, final position of the flaps) input by the pilot on a control panel. Simultaneously, this known device indicates to the pilot the instant when a regulatory distance since releasing the brakes has been reached, instructing him then to reduce the thrust from the engines (and therefore the noise resulting therefrom).
This known device therefore requires manual action by the pilots who has to input various values (speed, distance, altitude) into the control panel, with the risks of error which that may carry with it.
Furthermore, especially, the change of position of the lift-augmentation flaps is ordered and performed only when the landing gear has been fully raised, that is to say toward the end of the take-off phase.
The object of the present invention is to remedy these drawbacks. The invention relates to a system for control of lift-augmentation devices of an aircraft, making it possible automatically to optimize the position thereof during the phase of take-off by the aircraft.
To that end, said system of the type including:
controllable actuating means for shifting said lift-augmentation devices; and
a control unit suitable for generating control demands, in order to control said actuating means in such a way that the latter bring said lift-augmentation devices into a defined position, is noteworthy in that it further includes a first means for detecting the actual take-off by the aircraft and, if appropriate, for signaling such detection to the control unit, and in that, at the start of the take-off phase, said lift-augmentation devices are brought into a first position, in which they are deployed, and in that said control unit is formed in such a way as, at least when said first means signals the actual take-off, to generate a control demand making it possible to bring said lift-augmentation devices into a second position, in which they are retracted by comparison with said first position.
Thus, by virtue of the invention, during the take-off phase:
as long as the aircraft is rolling over the ground, the lift-augmentation devices (slats and/or flaps) are deployed in such a way as to augment the lift of the aircraft, which has the consequence of reducing the flight speeds and thus of reducing the runway length necessary for the take-off. Consequently, for a given type of aircraft, especially a civil transport airplane, which is equipped with the control system in accordance with the invention, it is possible either to increase its mass or to use a shorter take-off runway, by comparison with an aircraft of the same type not equipped with said control system; and
when the actual take-off has been achieved, that is to say when the wheels of the aircraft leave the ground, the lift-augmentation devices are brought into a less deployed position (that is to say with lift less augmented) in such a way as to reduce the drag, which then makes it possible to obtain a minimum angle of climb (with one failed engine) allowing a take-off in complete safety.
Furthermore, the control of the lift-augmentation devices is carried out automatically, without any intervention by the pilot of the aircraft, which allows the latter to concentrate exclusively on piloting, as the abovementioned aeronautical regulations require.
It will moreover be noted, in contrast to the control device disclosed by the abovementioned document U.S. Pat. No. 4,042,197, that the change of position (or of configuration), in accordance with the invention, of the lift-augmentation devices is not ordered toward the end of the take-off phase, but as soon as the aircraft leaves the ground, so as to reduce the drag immediately in order to optimize the minimum angle of climb and thus to achieve take-off in complete safety. Safety is not assured if the change of position is carried out toward the end of the take-off phase, as this known control device provides. Moreover, in the context of the present invention, the condition of xe2x80x9cactual take-offxe2x80x9d is the only essential condition (necessary and sufficient condition) for ordering the change of position, while the abovementioned known document still requires the speed of the aircraft to be taken into account.
According to the invention, said control system can be activated and de-activated by an operator of the aircraft, for example a pilot.
Moreover, advantageously, said first means detects a landing-gear-unloaded indication, in order to determine the instant when the aircraft actually takes off.
Furthermore, advantageously, for reasons of safety (stall limits), the control system in accordance with the invention further includes a first safety device which comprises:
means for determining the actual speed of the aircraft;
means for selecting a first minimum datum speed of the aircraft, for said second position of the lift-augmentation devices; and
means for comparing said actual speed with said first minimum datum speed, and said first safety device is associated with said control unit in such a way that the latter generates a control demand making it possible to bring the lift-augmentation devices into said second position only when said actual speed is greater than said first minimum datum speed.
According to the invention, said second position (retracted) of the lift-augmentation devices, especially depending on the configurations (or positions) which are available for the lift-augmentation devices, may be:
in a first embodiment, the position of the lift-augmentation devices for cruising flight of the aircraft; and
in a second embodiment, an intermediate position between said first position and a third position, in which the lift-augmentation devices are further retracted than in said second position.
In this second embodiment, the control system in accordance with the invention further advantageously includes a second means for detecting the start of the raising of at least one landing gear of the aircraft and, if appropriate, for signaling such a detection to the control unit, and said control unit is formed in such a way as to generate a control demand making it possible to bring said lift-augmentation devices from said second position to said third position, at least when said second means signals the start of the raising of the landing gear.
Furthermore, in this second embodiment, for reasons of safety or simply for the purposes of control, said control system further advantageously includes a second safety device which comprises:
means for determining the actual speed of the aircraft;
means for selecting a second minimum datum speed of the aircraft, for said third position of the lift-augmentation devices; and
means for comparing said actual speed with said second minimum datum speed, and said second safety device is associated with said control unit in such a way that the latter generates a control demand making it possible to bring the lift-augmentation devices into said third position only when said actual speed is greater than said second minimum datum speed.
Moreover, advantageously, said control system further includes a third means for determining whether a hydraulic flow rate which is sufficient to bring the lift-augmentation devices from the second position to the third position is available, and said control unit generates a control demand in order to bring said lift-augmentation devices from said second position to said third position only when a sufficient hydraulic flow rate is available.
Furthermore, advantageously, the control system in accordance with the invention may further include a means for verifying whether the second position is the most retracted position (or not), that is to say to verify whether it is possible to bring the lift-augmentation devices into a third position.